This invention relates to improved apparatus for conducting a stream of sample fluid to a detector that responds to a given characteristic of the sample fluid. Although it has application to many instruments, the invention has been found to be of considerable advantage when used with chromatographs which are used to measure the respective quantities of chemicals that are contained in a sample mixture. Basically, a chromatograph is comprised of a sample injector, a column, a detector, and it generally includes an output device or recorder and an integrator. The sample to be analyzed is injected by the injector into a stream of carrier fluid just as it enters the column and, ideally, each chemical contained in the sample is separated from the others so that they respectively elute from the other end of the column at different times. The elutants from the column flow through a detector that outputs an electrical signal corresponding to the value of a given characteristic of the fluid applied to it. For example, in a thermal conductivity cell, TC, the characteristic is the thermal conductivity of the sample gases; in a flame photometric detector, the characteristic is light emissions from sulphur molecules; and in a flame ionization detector, FID, the characteristic is the rate at which ions are produced as the gaseous chemical sample burns in the flame. When the carrier fluid passing through the detector contains no sample chemical, the detector output signal has a baseline value that may be offset from zero, but as separated sample chemicals pass through the detector, the value of the output signal changes so as to form a peak corresponding to the intensity of the characteristic of the sample chemical to which the detector is responsive. The greater the amount of the sample chemical, the larger is the peak. The integrator measures the area of the peak, i.e., the difference between each peak and the continuous baseline value, so as to give a measure of the amount of sample chemical present.
There are three types of noise generally present in the detector signal: low frequency wander and drift in the baseline value; higher frequency variations in the baseline value; and fluctuations in the baseline value that occur so many times during a peak that their net effect on the signal at the output of the integrator is virtually zero.
The effect of the low frequency wander and drift of the baseline can be virtually eliminated from the detector signal by alternately connecting the input of the detector to the column and a source of reference gas, and synchronously detecting or demodulating the output signal of the detector as suggested by John S. Craven and David E. Clouser in their U.S. patent application, Ser. No. 730,559, filed on Oct. 7, 1976, and entitled "Modulated Fluid Detector", now U.S. Pat. No. 4,254,654. If need be, make-up fluid that is the same as or similar to carrier fluid or reference gas as far as detector response is concerned may be added to the elutants from the column so as to increase their flow. While reference gas is flowing to the detector, the column elutant is vented to the atmosphere. During each cycle of alternation, the output signal of the detector varies from one peak value that occurs when the ratio of the concentration of sample gas to the concentration of reference gas contained within the detector is maximum to another peak value when the ratio is a minimum. The synchronous detector outputs a signal related to the difference in the detector signal resulting from the alternating flows of sample and reference gas.
In order to provide sufficient resolution, the frequency of the alternation at which the peak values of the electrical signal occur is chosen to be well above the highest frequency component of the peaks of sample chemical contained in the sample fluid eluting from the column. Inasmuch as this frequency of alternation is fast with respect to slow variation in the baseline value, adjacent peak values of the electrical signal produced by the detector are equally affected by slow variations in the baseline value so that when synchronous detection techniques are used, the effects of the slow variations of the baseline value are virtually eliminated by subtraction. The output signal of the synchronous detector may then be integrated so as to determine the amount of sample chemical corresponding to the peak of sample chemical. Unfortunately, however, because the sample fluid from the column is vented to the atmosphere during half of each cycle of alternation, the energy of the output signal of the detector in response to the sample chemical contained in the sample fluid is half what it would be if the elutant were allowed to flow through the detector continuously, but the noise energy due to the higher frequency baseline variations is the same so that the detector output-to-noise ratio is less than it might ideally be.